H,W = map(int,input().split())
S = [input().strip() for _ in range(H)]
ans = [S[0][0],[(0,0)]]
while True:
    T = ans[0]
    i,j = ans[1][0]
    if i+j==H+W-2:break
    ans[0] = T+"z"
    ans1 = []
    for i,j in ans[1]:
        if i==H-1:
            t = T+S[i][j+1]
            if t<ans[0]:
                ans[0] = t
                ans1 = [(i,j+1)]
            elif t==ans[0]:
                ans1.append((i,j+1))
        elif j==W-1:
            t = T+S[i+1][j]
            if t<ans[0]:
                ans[0] = t
                ans1 = [(i+1,j)]
            elif t==ans[0]:
                ans1.append((i+1,j))
        else:
            t = T+S[i][j+1]
            if t<ans[0]:
                ans[0]=t
                ans1 = [(i,j+1)]
            elif t==ans[0]:
                ans1.append((i,j+1))
            t = T+S[i+1][j]
            if t<ans[0]:
                ans[0]=t
                ans1 = [(i+1,j)]
            elif t==ans[0]:
                ans1.append((i+1,j))
    ans[1] = ans1
print(ans[0])